Method and apparatus for the preparation of silanes

ABSTRACT

The invention relates to a process for preparing dimeric and/or trimeric silanes by conversion of monosilane in a plasma and to a plant for performance of the process.

The invention relates to a process for preparing dimeric and/or trimericsilanes by reaction of monosilane with hydrogen in a plasma and to aplant for performance of this process.

In microelectronics, disilane is used for deposition of silicon layers,and this has to meet ultrahigh purity demands. However, the onlyprocesses known to date use catalysts. For instance, JP 02-184513discloses a process for preparing disilane using organometalliccatalysts based on platinum, rhodium or ruthenium complex catalystshaving organic phosphorus, arsenic or antimony ligands. These catalystscontribute to contamination in the ppb range of the disilane preparedand the disposal thereof is being viewed increasingly critically.

WO 2008/098640 A2 discloses a process for preparing hexachlorodisilane,which can be hydrogenated catalytically to disilane in a second processstep. This two-stage process is unsuitable for the inexpensivepreparation of high-purity disilane.

DE 36 39 202 discloses a further process for preparing disilane havingthe disadvantage of formation of significant amounts of elementalsilicon during the preparation of disilane. The reactor can only beoperated batchwise in this process and has to be cleaned in a costly andinconvenient manner after very short production times. A furtherdisadvantage lies in the high yield losses which arise firstly throughthe silicon deposition and secondly through losses of disilane ortrisilane because of stripping effects in the removal of hydrogen fromthe reaction products. These yield losses can be avoided in the case ofsynthesis via hexachlorodisilane, but the catalytic hydrogenation inturn results in contamination of the disilane and trisilane. The problemaddressed by the present invention was that of providing a process and aplant which avoid the stated disadvantages of the prior art andpreferably allow a continuous and preferably selective preparation ofdisilane and/or trisilane. In addition, it is to be possible to isolatedisilane and/or trisilane in high to ultrahigh purity. An additionalproblem was that of providing a particularly economically viable processon the industrial scale. These problems are solved by the processaccording to the invention and by the plant according to the inventionas per the features of Claims 1 and 12.

It has been found that, surprisingly, a gas phase treatment of areactant stream comprising monosilane under a given partial pressure ofthe monosilane in the gas mixture in the presence of hydrogen in anonthermal plasma at temperatures below 190° C. and preferably underreduced pressure leads selectively to formation of disilane and/ortrisilane.

The invention thus provides a process for preparing dimeric and/ortrimeric silanes of the general formula I where n=0 or 1, by

-   -   i) subjecting a reactant stream comprising monosilane of the        general formula II and hydrogen, especially a reactant stream        having a defined partial monosilane pressure in the gas mixture,

-   -   ii) to a gas discharge, preferably at a pressure between 0.05        mbar_(abs) and 15 000 mbar_(abs), more preferably under reduced        pressure, the gas discharge especially preferably corresponding        to a nonthermal plasma, and    -   iii) setting a defined ratio of the partial hydrogen pressure to        the partial pressure of the silanes which are gaseous under the        conditions selected in the resulting phase, wherein dimeric        and/or trimeric silanes of the formula I are obtained from the        resulting phase.

Preferably, in step iii), the dimeric and/or trimeric silanes of theformula I are obtained first, and these may be present in a mixture withhigher molecular weight silanes. Subsequently, a defined ratio of thepartial hydrogen pressure to the partial pressure of the silanes whichare gaseous under the conditions selected, especially of the monosilane,can be set in the resulting phase.

According to the invention, the setting of the partial pressures isaccomplished by means of a hydrogen-permeable membrane which ispreferably permeable only to hydrogen and is essentially impermeable tosilanes. Alternatively, it is likewise especially preferable when, instep iii), the dimeric and/or trimeric silanes of the formula I areobtained simultaneously in the resulting phase and a defined ratio ofthe partial hydrogen pressure to the partial pressure of the silaneswhich are gaseous under the conditions selected, especially of themonosilane, is set.

The invention thus provides a process for preparing dimeric and/ortrimeric silanes of the general formula I where n=0 or 1, i) wherein areactant stream comprising monosilane of the general formula II andhydrogen, especially having a defined partial monosilane pressure in thegas mixture, ii) is exposed to a nonthermal plasma at a pressure between0.1 mbar_(abs) and 1000 mbar_(abs), preferably at a pressure between 1mbar_(abs) and 950 mbar_(abs), and iii) a defined ratio of the partialhydrogen pressure to the partial pressure of the silanes which aregaseous under the conditions selected, especially to the partialpressure of the monosilane, is set in the resulting phase, and dimericand/or trimeric silanes of the formula I are obtained, where thepressure in process step iii) is elevated relative to the pressure inprocess stage ii).

The reactants used are high- to ultrahigh-purity monosilane and hydrogenwhich preferably each correspond to the impurity profile which follows.The total contamination in the monosilane or the hydrogen is from 100ppm by weight to 1 ppt by weight, especially to the detection limit,preferably less than or equal to 50 ppm by weight, further preferablyless than or equal to 25 ppm by weight. The total contamination includescontamination with boron, phosphorus and metallic elements other thansilicon. More preferably, the total contamination, in each caseindependently, for the monosilane and the hydrogen is, for the followingelements:

-   a. aluminium less than or equal to 15 ppm by weight to 0.0001 ppt by    weight, and/or-   b. boron less than or equal to 5 to 0.0001 ppt by weight, preferably    in the range from 3 ppm by weight to 0.0001 ppt by weight, and/or-   c. calcium less than or equal to 2 ppm by weight, preferably from 2    ppm by weight to 0.0001 ppt by weight, and/or-   d. iron less than or equal to 5 ppm by weight and 0.0001 ppt by    weight, preferably from 0.6 ppm by weight to 0.0001 ppt by weight,    and/or-   e. nickel less than or equal to 5 ppm by weight and 0.0001 ppt by    weight, preferably from 0.5 ppm by weight to 0.0001 ppt by weight,    and/or-   f. phosphorus less than or equal to 5 ppm by weight to 0.0001 ppt by    weight, preferably less than 3 ppm by weight to 0.0001 ppt by    weight, and/or-   g. titanium less than or equal to 10 ppm by weight, less than or    equal to 2 ppm by weight, preferably less than or equal to 1 ppm by    weight to 0.0001 ppt by weight, further preferably from 0.6 ppm by    weight to 0.0001 ppt by weight, especially preferably from 0.1 ppm    by weight to 0.0001 ppt by weight, and/or-   h. zinc less than or equal to 3 ppm by weight, preferably less than    or equal to 1 ppm by weight to 0.0001 ppt by weight, especially    preferably from 0.3 ppm by weight to 0.0001 ppt by weight,-   i. carbon and halogens together in a concentration which adds up to    the sum of concentrations a. to h. The value thus obtained is from    1000 ppm by weight to 1 ppt by weight, preferably from 100 ppm by    weight to 1 ppt by weight.

Preferably, the concentration of each impurity a. to h. is in the regionof the detection limit known to those skilled in the art, preferablythat of the GD-MS analysis method.

It has been found that, for a given reactant stream having a definedratio of hydrogen and silane, expressed in percent by volume (% byvol.), of preferably 15:1 to 1:5, preferably between 10:1 and 5:1, morepreferably between 10:1 and 8:1, further preferably about 90% by volumeof hydrogen and 10% by volume of monosilane, the best yields of disilaneand trisilane are obtained when the pressure in the gas dischargereactor is between 10 and 60 mbar_(abs). For instance, for a reactantstream of 90% by volume of hydrogen and 10% by volume of monosilane in anonthermal plasma at a pressure of 10 mbar_(abs), 0.7 g/h of disilanewas obtained in a continuous mode of operation, and at 20 mbar_(abs)even 0.75 g/h or at 25 mbar_(abs) 0.72 g/h. A very high yield of 0.85g/h of disilane can be isolated at 50 mbar_(abs). If the pressure isfurther increased slightly, the yield can be enhanced further. Foroptimal performance of the process, the reactant stream is preferablysubjected to a gas discharge in step ii) at a pressure between 5mbar_(abs) and 100 bar_(abs), more preferably between 7.5 mbar_(abs) and100 mbar_(abs), further preferably between 10 mbar_(abs) and 80mbar_(abs), preferably to a nonthermal plasma at a temperature between−60° C. and 10° C., especially at −40 to 0° C., further preferablyaround −10° C. plus/minus 5° C.

It is equally preferable when, in process step ii), the gas discharge isconducted at a pressure between 0.1 mbar_(abs) and 1000 mbar_(abs),preferably between 0.1 and 800 mbar_(abs), more preferably between 1mbar_(abs) and 100 mbar_(abs), a further preferred pressure range beingbetween 10 and 100 mbar_(abs), preferably between 10 and 80 mbar_(abs).It is further preferable when the gas discharge, especially thenonthermal plasma, is operated in step ii) at a temperature between −60°C. and 10° C. If the preferred pressure and temperature ranges areobserved during the plasma treatment of the reactant stream, it ispossible to selectively excite the Si—H bond to such an extent as toresult in silyl radical formation and subsequently dimerization of silylradicals. For selective silyl radical formation by excitation andcleavage of the Si—H bond, a mean electron energy of 5 eV in the weaklyionizing nonthermal plasma is required. For further chain extension, aninsertion of SiH₂ radicals into Si—H or Si—Si bonds of disilanesprobably takes place. In the event of too high an energy input in theregion of 12.3 eV, rather than selective radical formation, unwantedSiH₃ ⁺ ions would be formed, which lead to deposition of silicon onfurther breakdown. For high yields of disilane and trisilane, it istherefore crucial to optimize the process conditions in the nonthermalplasma for selective radical formation and the possibilities ofrecombination to higher silanes, and at the same time to suppress theformation of further decomposition products.

The disilane and/or trisilane formed can subsequently be condensed outvia a suitable temperature and pressure setting, especially in step iv),by adjusting the pressure by means of a compressor to a pressure above0.1 bar_(abs) to 100 bar_(abs), especially 1 bar_(abs) to 100 bar_(abs),preferably to 1 to 10 bar_(abs), within a temperature range from −60° C.to 20° C. For complete removal, preference is given to employing atwo-stage procedure in which, in a first component step iv.a), atemperature in the range from −20 to 10° C. is established in thecondenser at a pressure between 0.1 and 10 bar_(abs), especially between1 and 5 bar_(abs), and subsequently iv.b) the complete removal iseffected in the crude product vessel or crude product outlet atpreferably the same pressure at −60 to −20° C. by condensation of thedisilane and/or trisilane out of the resulting phase. This resultingphase is preferably contacted with a membrane permeable to hydrogen,such that a defined ratio of the partial hydrogen pressure to thepartial pressure of the silanes which are gaseous under the conditionsselected, especially of the unconverted monosilane, can be set. Theresulting phase which has been treated in this way, after the partialremoval of hydrogen, becomes the reactant stream again, and furthermonosilane may be metered into it before it is fed to the nonthermalplasma.

In this way, unconverted reactant of the general formula II can, ifrequired, be fed again to the nonthermal plasma. For full conversion ofthe monosilane used to disilane and/or trisilane of the general formulaI, the process is preferably operated as a cycle operation, by runningthrough process steps i), ii) and iii). The disilane and/or trisilane ofthe general formula I obtained by means of the reaction in thenonthermal plasma may already be obtained in pure form in the process.Further purification of the silanes can subsequently be effected bycontinuous or batchwise distillation. According to the product mixtureobtained, the distillative workup can be effected with one column orpreferably in a multicolumn system. In this context, it is readilyappreciable that trisilane forms preferentially as soon as disilane isconducted into the reactor again and/or is subjected to the gasdischarge for a prolonged period.

In this way, for example, disilane or trisilane can be isolated inultrahigh purity from the other reaction products and reactants. In the²⁹Si NMR spectrum, aside from the signal for the disilane, no furthercompounds are detectable. The contamination of the disilane or trisilanewith other metal compounds is at least less than or equal to 1000 ppb byweight down to below 100 ppt by weight. A particular advantage of thesilanes prepared by the process according to the invention is that theyare preferably free of chlorides or catalyst residues otherwisetypically used.

More preferably, the disilane or trisilane obtained is of ultrahighpurity and in each case has a sum total of total contamination of lessthan or equal to 100 ppm by weight down to the detection limit,especially to 1 ppt by weight, the total contamination preferably beingless than or equal to 50 ppm by weight. The total contamination isregarded as being contamination with boron, phosphorus and metallicelements other than silicon. More preferably, the total contamination ofthe disilane and/or trisilane for the following elements is less than orequal to:

-   a. aluminium less than or equal to 15 ppm by weight to 0.0001 ppt by    weight, and/or-   b. boron less than or equal to 5 to 0.0001 ppt by weight, preferably    in the range from 3 ppm by weight to 0.0001 ppt by weight, and/or-   c. calcium less than or equal to 2 ppm by weight, preferably between    2 ppm by weight and 0.0001 ppt by weight, and/or-   d. iron less than or equal to 5 ppm by weight and 0.0001 ppt by    weight, especially between 0.6 ppm by weight and 0.0001 ppt by    weight, and/or-   e. nickel less than or equal to 5 ppm by weight and 0.0001 ppt by    weight, especially between 0.5 ppm by weight and 0.0001 ppt by    weight, and/or-   f. phosphorus less than or equal to 5 ppm by weight to 0.0001 ppt by    weight, especially less than 3 ppm by weight to 0.0001 ppt by    weight, and/or-   g. titanium less than or equal to 10 ppm by weight, less than or    equal to 2 ppm by weight, preferably less than or equal to 1 ppm by    weight to 0.0001 ppt by weight, especially between 0.6 ppm by weight    and 0.0001 ppt by weight, preferably between 0.1 ppm by weight and    0.0001 ppt by weight, and/or-   h. zinc less than or equal to 3 ppm by weight, preferably less than    or equal to 1 ppm by weight to 0.0001 ppt by weight, especially    between 0.3 ppm by weight and 0.0001 ppt by weight,-   i. carbon and halogens together in a concentration which adds up to    the sum of concentrations a. to h. The value thus obtained is less    than or equal to 100 ppm by weight.

Preferably, the concentration of each impurity a. to i. is in the regionof the detection limit known to those skilled in the art. The totalcontamination with the aforementioned elements is preferably determinedby means of ICP-MS. Overall, the process can be monitored continuouslyby means of online analysis. The required purity can be checked by meansof GC, IR, NMR, ICP-MS, or by resistance measurement or GC-MS afterdeposition of the Si.

Additionally or alternatively to one of the aforementioned features, itis preferable when, in process step iii), the resulting phase is set toa pressure of 0.05 bar_(abs) to 100 bar_(abs), for example to 0.1 to 100bar_(abs), especially to a pressure between 1 bar_(abs) and 100bar_(abs), a further preferred pressure being between 0.5 or 1 bar_(abs)and 60 bar_(abs), and an especially preferred pressure being between 1and 10 bar_(abs). The hydrogen-permeable membrane used in the processand/or in the plant may preferably be a membrane comprising thefollowing materials: quartz, suitable metal, suitable metallic alloy,ceramic, zeolite, organic polymer and/or a composite membrane comprisingan at least two-layer structure with one or more of the aforementionedmaterials. In order to be suitable as material for thehydrogen-permeable membrane, it is necessary that the material, forexample quartz or palladium, has interstitial lattice sites, pores of adefined size, etc., through which hydrogen can diffuse or permeate andwhich are essentially impermeable to monosilane. A membrane usable withpreference may comprise, for example, a ceramic membrane having a layerstructure having a first microporous layer with pores smaller than 2 nm,adjoined by a mesoporous layer having pores between 3 and 10 nm, withoptional provision of another, macroporous layer having large pores upto 100 nm. It is preferable when the macroporous layer is a porousceramic material or a sintered metal.

Suitable membranes may preferably comprise the following materials:pore-free inorganic materials, for example tantalum, vanadium, niobium,aluminium, iron, palladium, or metal alloys, for example a palladiumalloy such as PdAl, PdCu, metal composites such as iron-copper as thinlayers in the thickness range of 10 to 100 nm, quartz and/or an organicsynthetic polymer, such as preferably hollow fibre membranes, in whichcase the membranes must preferably be permeable to hydrogen. Preferredhollow fibre membranes can be produced from polyamides, polyimides,polyamideimides or else from mixtures of these. If a palladium membraneis selected, it can be produced, for example, by chemical gas phasedeposition, electrochemical deposition, high-velocity flame spraying orphysical gas phase deposition, sputtering or what is called electronbeam vaporization.

Because of the high purity demands in relation to contamination withmetallic elements, preference is given to utilizing an ultrahigh-purityquartz membrane in the process and/or in the plant. This membrane shouldhave a pressure stability greater than 1 bar_(abs), preferably greaterthan 2 bar_(abs), more preferably greater than 3 bar_(abs), and maypreferably be applied to a porous Si support or alumina support. Thesame applies to palladium-based membranes, which may be produced from apalladium-aluminium or palladium-copper alloy, and preferably have apressure stability greater than 3 bar_(abs) on a porous Si support oralumina support. An advantageous aluminium composite membrane is onewhere the metal layer thickness is preferably below 100 nm.

The nonthermal plasma is generated in a plasma reactor in which aplasmatic conversion of matter is induced and is based on anisothermalplasmas. Characteristic features of these plasmas are a high electrontemperature Te of >10⁴ K and relatively low gas temperature TG of 10³ K.The activation energy needed for the chemical processes is exertedpredominantly through electron impacts (plasmatic conversion of matter).Typical nonthermal plasmas can be generated, for example, by glowdischarge, HF discharge, hollow cathode discharge or corona discharge.The working pressure at which the inventive plasma treatment isperformed is preferably between 1 and 2000 mbar_(abs), the phase to betreated preferably being set to a temperature of −40° C. to 190° C.,preferably to a temperature of −40° C. to 10° C. For a definition ofnonthermal plasma and of homogeneous plasma catalysis, reference is madeto the relevant technical literature, for example to “Plasmatechnik:Grundlagen und Anwendungen—Eine Einführung [Plasma Technology:Fundamentals and Applications—An Introduction]; collective of authors,Carl Hanser Verlag, Munich/Vienna; 1984, ISBN 3-446-13627-4”.

The invention likewise provides a plant, especially for performance ofthe aforementioned process, having a reactor for generation of a gasdischarge, with a dedicated upstream reactant feed and downstreamhydrogen-permeable membrane, in order to set a defined ratio of thepartial hydrogen pressure to the partial pressure of the gaseous silanesin the resulting phase. The membrane may also work at ambient pressure,in a diffusion-driven manner.

In addition to said reactor, the plant may also have one or more furtherreactors connected in series or in parallel. According to the invention,at least one reactor in the apparatus is an ozonizer. A great advantagelies in the alternative possible use of commercial ozonizers, such thatthe capital costs are lowered significantly. The reactors of theinvention are appropriately equipped with glass tubes, especially withquartz glass tubes, the tubes preferably being arranged in parallel orcoaxially and being spaced apart by means of inert material spacers.Suitable inert materials are especially Teflon, glass, and generallylow-K materials having a low dielectric constant. Materials having a lowdielectric constant are considered to be those whose dielectric constantis less than or equal to 9. Alternatively, the reactors may also beequipped not with glass tubes but with tubular dielectric componentswhich may have a higher dielectric constant with values between 1 and 10000, preferably 1.5 to 20.

It is known that the electron energy injected for the plasma discharge“E” is dependent on the product of pressure “p” and electrode spacing“d” (p d). For the process according to the invention, the product ofelectrode spacing and pressure is generally in the range from 0.001 to300 mm×bar, preferably from 0.05 to 100 mm×bar, more preferably 0.08 to0.3 mm×bar, especially 0.1 to 0.2 mm×bar. The discharge can be inducedby means of various kinds of AC voltages or pulsed voltages from 1 to10⁶ V. The curve profiles of this voltage may include rectangular,trapezoidal or pulsed profiles, or profiles composed piece by piece ofindividual profiles over time. Particularly suitable types are pulsedexcitation voltages, which enable simultaneous formation of thedischarge over the entire discharge space of the reactor. The pulseduration in pulsed operation is guided by the composition, the residencetime and the pressure of the reactant stream. It is preferably between10 ns and 1 ms. Preferred voltage amplitudes are 10 Vp to 100 kVp,preferably 100 Vp to 10 Vp, especially 50 to 5 Vp, in a microsystem. Thefrequency of the AC voltage may be set between 10 MHz and 10 ns pulses(duty ratio 10:1) down to low frequencies in the range from 10 to 0.01Hz. For example, an AC voltage having a frequency of 1.9 kHz and anamplitude of 35 kV peak-to-peak can be applied in the reactor. The powerinput is about 40 W.

The invention likewise provides a preferred plant, as illustrated inFIG. 1, which, downstream of the reactor 1, has a dedicated compressor 2to increase the pressure of the resulting phase, the compressor 2 moreparticularly being provided between the reactor 1 and the membrane 5.This compressor increases the pressure of the resulting phase after itleaves the reactor from about 60 mbar_(abs) to 2.5 bar_(abs). Thecompressed resulting phase is subsequently passed through a downstreamcondenser in order to condense the disilane and/or trisilane formed,while unconverted monosilane and hydrogen remain in the gas phase.

Thus, a particularly preferred plant 0 according to FIG. 1 has acompressor 2 downstream of the reactor 1, with a condenser 3 dedicatedto said compressor 2 and a downstream crude product outlet 4 or crudeproduct vessel 4 dedicated to said condenser 3. Further provideddownstream of the condenser 3, especially at or beyond the productvessel 4, is the membrane 5 for setting the partial hydrogen pressure ofthe resulting phase. The resulting phase is contacted with the membrane5 and then a reactant stream is obtained, which is transferred into thereactor by means of a dedicated line 11 upstream of the reactor. It ispossible to meter further monosilane from the monosilane source 9 intothis reactant stream, in order either to adjust the content ofmonosilane in % by volume or else to regulate the pressure of thereactant stream. A vacuum pump 6 dedicated to the reactor can beutilized for startup of the process and for regulation of the pressureduring the running reaction.

In a preferred embodiment, which is shown in FIG. 2, the crude productvessel or the crude product outlet is connected to a column 17,preferably a rectification column, for fractional distillation of thecrude product mixture. If appropriate, the column may have an upstreamproduct-conveying pump. Ultrahigh-purity disilane can be obtained as alow boiler at the top of the column, and ultrahigh-purity trisilane as ahigh boiler at the bottom.

A particularly preferred plant has an arrangement of the aforementionedplant parts, in order to enable the performance of a cycle operation ofthe aforementioned process. In this plant, the reactor 1 has a dedicateddownstream compressor 2, as illustrated in FIG. 1. Said compressor has adedicated condenser 3, and the plant has the hydrogen-permeable membrane5 downstream of the condenser 3, with a line 12 dedicated to one side ofthe membrane 5 and to the reactor 1, and a product outlet 4 or productvessel 4 is also provided downstream of the condenser 3; and dischargedhydrogen is removed through a further line 15, which may have adedicated inert gas line for hydrogen removal, on the other side of themembrane 5.

The example which follows illustrates the process according to theinvention in detail.

EXAMPLE 1

Monosilane is vaporized continuously from a pressurized gas bottle 9 bymeans of a pressure regulator via the reactant feed 12 into the reactor1 and conducted through a gas discharge zone comprising dielectric. Thenonthermal plasma is operated in the reactor at −10° C. and at 60mbar_(abs). The Si—H bond of the monosilane in the reactant streamcomposed of 10% by volume of monosilane and 90% by volume of hydrogen isselectively excited to form silyl radicals, which react to form disilaneor trisilane and form the resulting phase. After increasing the pressureof the resulting phase to about 2.5 bar_(abs), it is passed through acondenser 3 cooled to about 0° C., in order to condense disilane andtrisilane, which can run off into the crude product vessel 4 which is ata controlled temperature of −40° C. The remaining gaseous resultingphase is run past one side of the membrane 5 through a line 10. Hydrogenin the resulting phase diffuses through the membrane 5 and can beremoved via the line 15. At the membrane, a defined ratio of the partialhydrogen pressure to the partial pressure of the monosilane which isgaseous under the conditions selected is set in the resulting phase. Asa result of this measure, the resulting phase becomes a reactant streamwhich is fed again to the gas discharge zone comprising dielectric inthe reactor, optionally after metered addition of further monosilane. Inthe crude product vessel, disilane is enriched in the mixture having aproportion of trisilane, which are pumped by the product pump 16 to thedistillation column 17, in order to be fractionally distilled therein.

By continuous fractional distillation, ultrahigh-purity disilane wasdrawn off as a low boiler at the top of the column 17 and trisilane as ahigh boiler at the bottom of the column.

The general process regime of Example 1 is not limited to the specifiedprocess parameters, but can be generalized in accordance with thedescription.

FIG. 1 and FIG. 2 show a schematic diagram of an inventive plant 0 forperformance of the process according to the invention.

FIG. 3 shows a diagram of the hydrogen permeability of various membranematerials.

LIST OF REFERENCE NUMERALS

-   0 plant-   1 reactor-   2 compressor-   3 condenser-   4 crude product outlet or crude product vessel-   5 membrane-   6 vacuum pump-   7 inverter for plasma production-   8 hydrogen source—startup of the process-   9 monosilane source-   10 line/resulting phase-   11 line/reactant feed-   12 line/reactant feed-   13 line/monosilane-   14 line/resulting phase-   15 line/hydrogen-   16 product-conveying pump-   17 column—fractional distillation-   18 line—inert gas for hydrogen removal

1. A process for preparing dimeric and/or trimeric silanes of thegeneral formula I:

where n=0 or 1, comprising: i) subjecting a reactant stream comprisingmonosilane of the general formula II

 and hydrogen, ii) to a gas discharge, and iii) obtaining dimeric and/ortrimeric silanes of the formula I from the resulting phase, and settinga defined ratio of the partial hydrogen pressure to the partial pressureof the silanes which are gaseous under the conditions selected in theresulting phase.
 2. The process according to claim 1, wherein thepressure in process step iii) is elevated relative to the pressure inprocess stage ii).
 3. The process according to claim 1, wherein theresulting phase in process step iii) has a pressure of 1 bar_(ab) to 100bar_(abs).
 4. The process according to claim 1 wherein the monosilane inprocess step ii) is subjected to the gas discharge in the presence ofhydrogen at a pressure between 0.05 mbar_(abs) and 15,000 mbar_(abs). 5.The process according to claim 1, wherein the gas discharge in processstep ii) is effected at a pressure between 0.1 mbar_(abs) and 1,000mbar_(abs).
 6. The process according to claim 1 wherein the gasdischarge in process step ii) is effected at a temperature between −60°C. and 10° C.
 7. The process according to claim 1 wherein the reactantstream has a defined ratio of hydrogen and monosilane in percent byvolume (% by vol.) of 15:1 to 1:5.
 8. The process according to claim 1,wherein the reactant stream in step ii) is exposed to a nonthermalplasma.
 9. The process according to claim 1, wherein the defined ratioin process step iii) of the partial hydrogen pressure to the partialpressure of the gaseous silanes is set by means of a hydrogen-permeablemembrane.
 10. The process according to claim 9, wherein the membrane ispermeable to hydrogen and essentially impermeable to silanes.
 11. Theprocess according to claim 9, wherein said membrane comprises at leastone of the following materials: quartz, metal, metallic alloy, ceramic,zeolite, organic polymer and/or a composite membrane having an at leasttwo-layer structure comprising one or more of the aforementionedmaterials.
 12. A plant for performance of the process according to claim1 comprising: a reactor for generation of a gas discharge, with adedicated upstream reactant feed and downstream hydrogen-permeablemembrane, in order to set a defined ratio of the partial hydrogenpressure to the partial pressure of the gaseous silanes in the resultingphase.
 13. The plant according to claim 12, wherein the reactor has adedicated downstream compressor that increases the pressure of theresulting phase, the compressor being provided between the reactor andthe membrane.
 14. The plant according to claim 12, that comprises acompressor downstream of the reactor, with a condenser dedicated to saidcompressor and a downstream crude product outlet or crude product vesseldedicated to said condenser, downstream of which is disposed themembrane for setting the partial hydrogen pressure of the resultingphase by contacting of the resulting phase with the membrane, whichgives a reactant stream which is transferred by means of a line into thereactor.
 15. The plant according to claim 12 having an arrangementwherein the reactor has a dedicated downstream compressor, and saidcompressor has a dedicated condenser, and the plant has thehydrogen-permeable membrane downstream of the condenser, with a linededicated to one side of the membrane and to the reactor, and a productoutlet or product vessel is also provided downstream of the condenser;and discharged hydrogen is removed on the other side of the membrane.